Which of the following statements are correct w.r.t. unit factors described by Mendel?
(a) These factors serve as the basic units of heredity.
(b)  They are passed unchanged from generation to generation.
(c) They occur in pairs.

Mendel suggested that these factors serve as the basic units of heredity and are passed unchanged from generation to generation, determining various traits expressed by each individual plant.
Each diploid individual receives one factor from each parent and therefore unit factors occurs in pairs.